This invention relates to a process for decomposing a polymer which is capable of undergoing thermal depolymerization to its monomer or monomers, and for recovery of at least one of the monomers, which monomer may be recycled to a polymerization process.
The conventional decomposition or depolymerization process for the depolymerization of waste poly(methyl metharcylate) ("PMMA") makes use of a lead bath reactor. The waste PMMA is crushed into chips (1 to 5 cm in diameter) and decomposed on the surface of the molten lead bath. The molten lead is maintained at 520 to 550.degree. C. by means of a diesel burner which operates at about 900.degree. C. In the reactor, depolymerization takes place producing a gaseous product which is (methyl methacrylate) monomer ("MMA"), which is condensed in a condenser, with a solid dross or ash remaining on the surface of the lead bath. The dross is composed mainly of carbonaceous material, lead (40-60% m/m Pb) and inorganic residues from pigments and additives. The crude monomer (approximately 85% MMA) may then be purified as follows. The monomer is washed in a 9% caustic solution (containing 40 ppm copper sulphate heptahydrate CuSO.sub.4.7H.sub.2 O) which removes any traces of lead. The washings (containing approximately 12 ppm lead) are treated and discharged to a slimes dam, but represent a potential environmental hazard. The final purification step is a vacuum distillation at 65.degree. C., to remove heavy organic impurities. This type of depolymerization reactor may be operated to produce refined monomer with an average purity of 99.3% and yield of 85%.
This type of depolymerization reactor has numerous disadvantages. The most significant disadvantage of the lead bath reactor is the environmental and safety hazard associated with lead. Another disadvantage of the reactor is that it has to be operated on a non-continuous basis. Generally, the reactor is shut down after approximately five days operation due to fouling on the surface caused by the dross, which inhibits heat transfer from the lead to the PMMA. The cooling and cleaning operation may result in one co two days downtime. A further disadvantage is that the lead containing dross is generated as a waste product. Lead must be recovered from the dross, which then needs to be disposed of in an environmentally suitable manner. The lead bath subsequently must be reheated to operating temperature. Thus, this process is very energy inefficient.
In an article entitled Polymethyl Methacrylate Binder Removal from an Alumina Compact:Microwave versus Conventional Heating in Mat. Res. Soc. Symp. Proc. vol. 269, 1992 by moore et al, there is disclosed that compact samples of alumina and polymethyl methacrylate were heated in a 2450 MHz microwave cavity and by conventional heating in an electric furnace. Various heating schedules were used to effect the removal of the polymeric binder by thermal decomposition. Dielectric properties, porosity and other physical properties were investigated in order to better understand the binder removal process in a microwave field. Results of the study emphasized the amount of the carbon residuals remaining in the bulk. In this article it is stated that PMMA decomposes into monomer, water. benzene and other components. These components are then further decomposed to hydrocarbons between 500.degree. C. and 1000.degree. C. In other words, the PMMA is completely decomposed into hydrocarbons.
In an article entitled Microwave reactions of polyethylene terephthalate, in Polym.Mater.Sci.Eng, 71,531-2, 1994, by Gilmer et al, there is disclosed that a glass reactor was designed for the microwave cavity to allow conventional chemical reactions to be carried out with focused microwave heating. Employing this setup for reactions concerning the synthesis and depolymerization of poly(ethylene terephthalate) (PET), in general, the monomers for PET are not good microwave heaters (absorbers), with the possible exception of ethylene glycol (EG). In the depolymerization of PET by EG and the reaction of EG with di-methyl terephthalate to form bis (hydroxyethyl) terephthalate and methanol (MeOH), identical reaction rates were obtained using either thermal or microwave heating.
In other words, this article discloses in general the depolymerization of PET, in a solvent, i.e EG.
There is thus a need for a new process for decomposing a polymer to its monomer or monomers, which process makes use of an economic, safe and environmentally friendly thermal decomposition route which eliminates the use of lead, and which results in a lesser amount of "ash" or "dross" which is free of contamination by lead.